Radio frequency filters are employed to implement high-frequency circuits in e.g. base stations of mobile telephone networks. Filters may be used e.g. as interface circuits and filtering circuits in amplifiers of transmitters or receivers in base stations.
Resonator filters comprising a shell construction, or body, are of several types, e.g. coaxial resonator filters. In coaxial resonator filters, the shell construction envelops a conductor which is positioned in a section of the shell construction, i.e. a resonator cavity, and which is called a resonator or resonator rod. High-frequency filters, particularly more complex filters, are provided with a multi-section shell construction and what is known as a subdivision, or zoning. In this case, the resonator filter has a multi-section, or multicavity, shell construction, in other words, it comprises a plurality of resonator cavities, or sections in the shell construction, each of which constitutes a separate resonance circuit with the corresponding resonator, making the filter thus multi-circuit.
Coupling adjusting elements for adjusting the strength of the coupling between resonance circuits are used in filters in the area between the lower ends, or the inductive ends of the resonators. A coupling adjusting element is of a conductive material, and helps the resonators "see" each other more strongly, even though the coupling element does not even connect the resonators, since coupling takes place inductively via a magnetic field and since the position of the conductive coupling element in the area between the resonators shortens the length of the area between the resonators, i.e. the area which is free from conductive material.
Filters also employ frequency tuning elements.
The operating frequency, or resonance frequency, of a resonance circuit composed of a resonator and a section is tuned in order to make the resonance circuit operate in the desired manner, whereby a resonance circuit alone or, in practice, an integral unit composed of a plurality of resonance circuits, will implement a desired frequency response, which for example in the case of a bandpass filter is the passband, the signals inside of which the filter lets through.
It is known that the resonance frequency of a resonance circuit of a filter is tuned by changing the distance between the free end of the resonator and the grounded shell by means of a frequency tuning element, a shortened distance making the capacitance between the free end of the resonator and he shell increase and the resonance frequency decrease, whereas a longer distance makes the capacitance decrease and the resonance frequency increase.
Some known resonator filters are so manufactured that the shell construction and the resonators are manufactured from separate parts, whereby the resonators are e.g. soldered onto the bottom or cover of the shell construction, i.e. either end of the shell construction. Such a construction increases the probability of harmful intermodulation, and is slow to manufacture. Solutions are also known in which material is milled from a sufficiently large metal block, the remaining part of the block constituting the shell construction and resonator rods of the filter. Such a solution consumes much raw material and requires time-consuming manufacturing steps.
U.S. Pat. No. 4,706,051 discloses a solution according to which halves of a waveguide shell construction are manufactured by forging into a die, a slug of material is hit by a punch such that the material is displaced in the closed space between the die and the punch. This publication does not disclose any solution for manufacturing resonators. The solution has drawbacks, since it involves the manufacture of complementary halves of a shell, and since the slug material displaced as a result of punching to form a half of the shell construction does not flow freely, the closed die restricting the flow of the material.
U.S. Pat. No. 5,329,687 discloses a solution according to which both a shell construction and a resonator are molded or extruded from plastic as an integral unit to be coated with metal. However, the thermal conductivity of such a construction is not good. In addition, U.S. Pat. No. 4,278,957 discloses a solution according to which resonators are cast in the shell construction. The construction of the latter publication is manufactured by die casting, which requires a multi-element die arrangement which must open in at least three directions. On account of the material residues left in the joints of the die, a resonator made by die casting will not be entirely circular, which impairs the electrical properties of the resonator.
A construction made from thin sheet by punching or machining is known, in which the shell, the resonator and an adjusting projection for adjusting the inter-resonator coupling, are all an integral unit, i.e. made from the same thin sheet as one piece, which is bent into the shape of a shell. The problem is that the free ends of the wall portions to be bent have to be soldered in order to prevent leakage points. Since a thin sheet is involved, said technique does not necessarily provide such material strengths for the shell and the resonator which are required by some applications consuming power and requiring a strong wall construction and resonator.
Known coupling adjusting elements for adjusting the strength of the coupling between resonance circuits are whiskers, threads or other projections soldered or otherwise fastened to the resonator or the shell construction in the area between the resonators.
Known resonance circuit frequency tuning elements are tuning bolts placed in the end of the shell construction, such as the cover, or elsewhere in the shell construction. It is common to said tuning elements that they are separate parts with respect to the resonators and the shell construction and have to be fastened separately to the resonator or the filter, which increases the number of components and slows down and complicates the manufacture.